Steel sheet for cans and method for manufacturing the same

Abstract

The present invention provides a steel sheet for cans that possesses the corrosion resistance necessary for cans, as well as processability for curled sections and pressure resistance for can lids, and a method for manufacturing the same. The metal has a specified component composition, with a Mo content of 0.013% by mass or less as precipitated Mo, containing a ferrite phase of 70% or more by area ratio, a martensite phase of 5% to 22% by area ratio, and a metal structure in which the proportion of unrecrystallized ferrite is 3% or less by area ratio of the total structure, and the yield strength YS (MPa) and tensile strength TS (MPa) are as shown in (1) below. (YS+TS) / 2 ≥ 470MPa ……(1) A steel sheet for cans that satisfies the following conditions and has a total elongation of 10% or more.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for cans excellent in processability of can lids and pressure resistance of can lids, and a method for producing the same. [Background Art]

[0002] In recent years, in order to reduce costs for steel sheets for cans, thinning of steel sheets through higher strength has been promoted. Specifically, application of high-strength thin steel sheets with yield strength (YP) of 450 MPa or more to cans has been studied. When such high strength is required, instead of SR (Single Reduce) material which is rolled only once and conventionally used for steel sheets for cans, DR (Double Reduce) material which is rolled once more after annealing may be used. However, although DR material can achieve high strength by being rolled twice, its ductility decreases, so for example, when processed into a can lid, wrinkles are likely to occur at the curled portion around the can lid. In addition, can lids are required to have pressure resistance strength such that they do not deform during retort sterilization after filling contents, but as the steel sheet is thinned due to higher strength, the lid tends to deform easily. Furthermore, since DR material is rolled twice, it is more costly than SR material.

[0003] In response to such requirements, for example, Patent Document 1 discloses a composition containing, by mass%: C: 0.085% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 0.60% or less, P: 0.02% or less, S: more than 0.010% and 0.020% or less, Al: 0.02% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Nb: 0.007% or more and 0.030% or less, B: 0.0010% or more and 0.0050% or less, wherein B / N, which is the ratio of the B content (mass%) to the N content (mass%), is 0.80 or more, with the balance being iron and unavoidable impurities, the steel sheet for cans has a ferrite structure containing pearlite in an area ratio of 1.0% or more, has a yield stress of 500 MPa or more, a tensile strength of 550 MPa or more, a uniform elongation of 10% or more, and a yield elongation of 5.0% or less, and has high strength and excellent workability.

[0004] Patent Document 2 discloses a steel sheet for cans, having a composition in mass%, consisting of C: 0.03-0.13%, Si: 0.03% or less, Mn: 0.3-0.6%, P: 0.02% or less, Al: 0.1% or less, N: 0.012% or less, and further containing one or more of Nb: 0.005-0.05%, Ti: 0.005-0.05%, and B: 0.0005-0.005%, with the remainder being iron and unavoidable impurities, and having a ferrite structure with a cementite content of 0.5% or more, an average ferrite grain size of 7 μm or less, a tensile strength of 450-550 MPa after paint baking treatment, a total elongation of 20% or more, and a yield elongation of 5% or less.

[0005] Patent Document 3 describes a material containing, by mass ratio, C: 0.020-0.150%, Si: 0.05% or less, Mn: 1.00%, P: 0.050% or less, S: 0.010% or less, N: 0.0100% or less, Al: 0.100% or less, and Nb: 0.005-0.025%, with the remainder being iron and unavoidable impurities, resulting in a substantially single-phase ferrite structure with a yield strength of 40 kgf / mm². 2 In summary, steel sheets for cans with an average grain size of 10 μm or less and a plate thickness of 0.300 mm or less are... It has been disclosed.

[0006] Patent Document 4 describes a material containing, by mass%, C: 0.0010~0.10%, Si: 0.04% or less, Mn: 0.10~0.80%, P: 0.007~0.100%, S: 0.10% or less, Al: 0.001~0.100%, and N: 0.0010~0.0250%, with the remainder being Fe and unavoidable impurities. The difference between the dislocation density at the outermost layer and the dislocation density at a depth of 1 / 4 of the plate thickness from the surface is 1.94 × 10¹⁴ m². -2 The following high-strength steel plates for containers are disclosed, having a tensile strength of 400 MPa or more and a fracture elongation of 10% or more. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2020 / 105406 [Patent Document 2] Japanese Patent Publication No. 2008-274332 [Patent Document 3] Japanese Patent Application Publication No. 8-325670 [Patent Document 4] International Publication No. 2015 / 166653 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the steel sheet for cans described in Patent Document 1 has a lower limit of uniform elongation of 10%, and at such a ductility level, the processability of can lids and the compressive strength are unknown.

[0009] While Patent Document 2 mentions strength and elongation, it does not evaluate the processability or pressure resistance of the can lid, and therefore the superiority or inferiority of the can lid in terms of processability and pressure resistance remains unclear.

[0010] Although Patent Document 3 proposes a steel with a balanced combination of high strength and ductility achieved by optimizing the steel structure, it completely fails to consider the curling properties of can lids or the required pressure resistance. Therefore, it is difficult to obtain a steel sheet that satisfies the required workability and desired pressure resistance of the curled portion of a can lid using the steel sheet for cans and its manufacturing method described in Patent Document 3.

[0011] Patent Document 4 describes evaluating the pressure resistance by forming a can lid using a high-strength steel sheet for containers that has a tensile strength of 400 MPa or more and a fracture elongation of 10% or more. However, in Patent Document 4, the formability of the can lid is evaluated using the Erichsen value, and the processability of the curled portion is not considered at all. Therefore, it is difficult to obtain a good curled portion of a can lid using the technology described in Patent Document 4.

[0012] The present invention has been made to solve the problems of the prior art described above, and aims to provide a steel sheet for cans and a method for manufacturing the same, which has the corrosion resistance necessary for a steel sheet for cans, as well as processability of the curled portion and pressure resistance strength for can lids. [Means for solving the problem]

[0013] The inventors diligently conducted research to solve the above problems. As a result, they found that by adjusting the steel composition and amount of precipitated Mo, the ratio of ferrite phase and martensite phase in the metal structure, the amount of unrecrystallized ferrite, and the relationship between yield strength (YS) and tensile strength (TS), a high-strength steel sheet with excellent curl-forming properties and compressive strength when processed into a can lid could be obtained. In particular, they found that controlling the amount of precipitated Mo could suppress the occurrence of wrinkles in the curled portion.

[0014] Furthermore, we found that precisely controlling the heating temperature, finishing temperature, and winding temperature in the hot rolling process, as well as the heating rate, holding time, cooling rate after annealing, and cooling stop temperature in the annealing process, is important for adjusting the amount of precipitated Mo, the proportion of the ferrite phase, the proportion of the martensite phase, and the amount of unrecrystallized material.

[0015] This invention was completed as a result of further investigation based on the above-mentioned findings by the inventors, and its gist is as follows.

[0016] [1] In mass%, C: 0.05% to 0.13%, Si: 0.01% to 0.04%, Mn: 0.1% to 0.6%, P: 0.02% or less, S: 0.02% or less, Al: 0.01% to 0.10%, N: 0.0005% to 0.0080%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, Mo: 0.010% to 0.050%, B: 0.0005% to 0.0050%, Cu: 0.003% to 0.500%, Cr: 0.003% to 0.300%, N The composition has the following components: i: 0.005% to 0.500%, V: 0.005% to 0.100%, Sn: 0.003% to 0.050%, with the remainder being iron and unavoidable impurities; the Mo content as precipitated Mo is 0.013% by mass or less; it contains a ferrite phase of 70% or more by area ratio and a martensite phase of 5% to 22% by area ratio; the proportion of unrecrystallized ferrite is 3% or less by area ratio of the total structure; and the yield strength YS (MPa) and tensile strength TS (MPa) are given by the following formula (1) (YS+TS) / 2 ≥ 470MPa ……(1) A steel sheet for cans that satisfies the following conditions and has a total elongation of 10% or more.

[0017] [2] A method for manufacturing steel sheets for cans as described in [1], comprising: a hot rolling step in which a steel slab having the above-mentioned component composition is heated to 1150°C or higher and hot-rolled under conditions that the finishing temperature is 800°C or higher and 950°C or lower and the winding temperature is 550°C or higher and 750°C or lower; a cold rolling step in which the hot-rolled sheet having undergone the hot-rolling step is cold-rolled to a reduction ratio of 85% or higher; and an annealing step in which the cold-rolled sheet having undergone the cold-rolling step is annealed to an annealing temperature of 640°C or higher and 860°C or lower at an average heating rate of 5 to 40°C / second. A method for manufacturing steel sheets for cans, comprising: an annealing step of raising the temperature, holding it at the annealing temperature for 10 seconds or more and 90 seconds or less, primary cooling to a cooling stop temperature of 250°C or more and 350°C or less at an average cooling rate of 10°C / second or more and 230°C / second or less, and secondary cooling to a cooling stop temperature of 150°C or more and 200°C or less at an average cooling rate of 0.2°C / second or more and 10.0°C / second or less; and a temper rolling step of subjecting the annealed sheet that has undergone the annealing step to temper rolling with a temper rolling ratio of 0.5% or more and 5.0% or less. Effects of the Invention

[0018] According to the present invention, it is possible to provide a steel sheet for cans which has the corrosion resistance required as a steel sheet for cans, and is excellent in curling workability and compressive strength for can lids. Since the steel sheet for cans of the present invention is excellent in ductility, it is suitable as a steel sheet for cans formed into complicated shapes, for example, a steel sheet for can lids subjected to curling processing. Furthermore, by applying the steel sheet produced by the method for producing a steel sheet for cans of the present invention to cans, further higher strength and weight reduction can be achieved, which greatly contributes to the development of industry. Mode for Carrying Out the Invention

[0019] <Steel sheet for cans> A steel sheet for cans according to one embodiment of the present invention will be described below. Component Composition First, the appropriate range and the reason for limitation of the component composition of the steel sheet for cans of the present embodiment will be described. Unless otherwise specified, "%" representing the component composition below means "% by mass". In addition, cases excellent in curling workability are also simply referred to as being excellent in workability. - C: 0.05% or more and 0.13% or less C is an element contributing to strength, and precipitates as a solid solution or carbide in steel, and has the effect of increasing the strength of steel. In order to obtain a desired yield strength by utilizing these effects, it is necessary to contain 0.05% or more of C. Therefore, the C content is 0.05% or more, preferably 0.06% or more, and more preferably 0.07% or more.

[0020] Further, when C is excessively contained, the proportion of martensite phase exceeds 22% and the proportion of ferrite phase becomes less than 70%, which reduces ductility, and the proportion of unrecrystallized ferrite exceeds 3%, so that wrinkles occur when the steel sheet is processed into the curled portion of a can lid. To prevent this, the C content is 0.13% or less, preferably 0.12% or less, and more preferably 0.11% or less. - Si: 0.01% or more and 0.04% or less Si contributes to increasing the strength of steel through solid solution strengthening. To obtain the desired yield strength by utilizing these effects, the Si content must be 0.01% or more. Therefore, the Si content should be 0.01% or more.

[0021] Furthermore, if the Si content exceeds 0.04%, there is a risk of serious problems with corrosion resistance and surface properties. To prevent this, the Si content should be 0.04% or less, preferably 0.03% or less. ·Mn: 0.1% or more and 0.6% or less Mn is an element that contributes to strength, and it contributes to increasing the strength of steel by being dissolved in solid solution. As is the objective of this invention, in order to obtain the desired compressive strength, it is necessary to contain 0.1% or more of Mn. For this reason, the Mn content is set to 0.1% or more, and preferably to 0.4% or more.

[0022] Furthermore, if the Mn content exceeds 0.6%, the proportion of the martensite phase will exceed 22% and the proportion of the ferrite phase will fall below 70%, resulting in decreased ductility. Additionally, the proportion of unrecrystallized ferrite will exceed 3%, causing wrinkles to form when the steel sheet is processed into the curled portion of a can lid. To prevent this, the Mn content should be 0.6% or less. ·P:0.02% or less P is an element with high solid solution strengthening ability. To obtain this effect, it is preferable to contain P at a concentration of 0.001% or more.

[0023] Furthermore, if the P content exceeds 0.02%, the corrosion resistance deteriorates. To prevent this, the P content should be 0.02% or less, preferably 0.007% or more, and more preferably 0.018% or less. ·S: 0.02% or less S is an unavoidable component in steel, forming coarse inclusions such as MnS, which significantly reduces local ductility. To prevent this, the S content should be 0.02% or less, preferably 0.018% or less.

[0024] Furthermore, reducing the sulfur content to less than 0.001% would incur excessive costs in steel refining. For this reason, it is preferable for the sulfur content to be 0.001% or more, and more preferable for it to be 0.007% or more. ·Al: 0.01% or more and 0.10% or less Al acts as a deoxidizing agent, and to achieve this effect, the product must contain 0.01% or more Al. Therefore, the Al content should be 0.01% or more, preferably 0.03% or more.

[0025] Furthermore, if the Al content exceeds 0.10%, surface defects may occur in the steel sheet, and manufacturing costs will increase. To prevent this, the Al content should be 0.10% or less, preferably 0.08% or less. ·N: 0.0005% or more and 0.0080% or less N combines with carbonitride-forming elements such as Al, Nb, and Ti to form precipitates, contributing to improved strength. To achieve this effect, the N content should be 0.0005% or more, preferably 0.0006% or more.

[0026] Furthermore, if the N content exceeds 0.0080%, the ductility decreases. To prevent this, the N content should be 0.0080% or less, preferably 0.0075% or less. ·Ti: 0.005% or more and 0.030% or less Ti is one of the important additive elements in this invention because it combines with C and N to form carbonitrides, contributing to improved strength. Furthermore, the addition of Ti suppresses the formation of BN and enhances the strength-improving effect due to grain boundary segregation of B. To obtain this effect, it is necessary to contain 0.005% or more Ti. For this reason, the Ti content is set to 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0027] Furthermore, if the Ti content exceeds 0.030%, the strength increases, leading to a decrease in ductility, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles to occur when the steel sheet is processed into the curled portion of a can lid. To prevent this, the Ti content should be 0.030% or less, preferably 0.028% or less, and more preferably 0.025% or less. ·Nb: 0.005% or more and 0.030% or less Nb is one of the important additive elements in this invention because it combines with C and N to form carbonitrides, contributing to improved strength. To obtain this effect, it is necessary to contain 0.005% or more Nb. Therefore, the Nb content is set to 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0028] Furthermore, if the Nb content exceeds 0.030%, the strength increases, leading to a decrease in ductility, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles to form when the steel sheet is processed into the curled portion of a can lid. To prevent this, the Nb content should be 0.030% or less, preferably 0.028% or less, and more preferably 0.025% or less. ·Mo: 0.010% or more and 0.050% or less Mo contributes to strength improvement by precipitating in steel as a solid solution or carbide. To obtain this effect, the steel needs to contain 0.010% or more of Mo. Therefore, the Mo content should be 0.010% or more, preferably 0.012% or more, and more preferably 0.015% or more.

[0029] Furthermore, excessive Mo content increases the yield strength and decreases ductility, and the amount of precipitated Mo (described later) exceeds 0.010%, causing wrinkles to occur when the steel sheet is processed into the curled portion of a can lid. To prevent this, the Mo content should be 0.050% or less, preferably 0.047% or less, and more preferably 0.045% or less. ·B: 0.0005% or more and 0.0050% or less B contributes to strength improvement by segregating at grain boundaries. To obtain this effect, the material must contain 0.0005% or more of B. Therefore, the B content should be 0.0005% or more, preferably 0.0008% or more.

[0030] Furthermore, if B is included in a large amount exceeding 0.0050%, its effect will saturate. For this reason, the B content should be 0.0050% or less, preferably 0.0040% or less. ·Cu: 0.003% or more and 0.500% or less Cu has the effect of solid-solubilizing in steel or improving hardenability, and contributes to improving the pressure resistance of the lid; therefore, it is one of the important additive elements in this invention. To obtain this effect, it is necessary to contain 0.003% or more of Cu. For this reason, the Cu content is set to 0.003% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0031] Furthermore, if the copper content is excessive, the proportion of the martensite phase will exceed 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the copper content should be 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less. ·Cr: 0.003% or more and 0.300% or less Cr has the effect of improving hardenability and contributes to improving the pressure resistance strength of the lid, and is therefore one of the important additive elements in this invention. In order to obtain this effect, it is necessary to contain 0.003% or more of Cr. For this reason, the Cr content is set to 0.003% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0032] Furthermore, if the Cr content is excessive, the proportion of the martensite phase will exceed 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the Cr content should be 0.300% or less, preferably 0.270% or less, and more preferably 0.250% or less. ·Ni: 0.005% or more and 0.500% or less Ni has the effect of improving hardenability and contributes to improving the pressure resistance strength of the lid, and is therefore one of the important additive elements in this invention. To obtain this effect, it is necessary to contain 0.005% or more Ni. For this reason, the Ni content is set to 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0033] Furthermore, if the Ni content is excessive, the proportion of the martensite phase will exceed 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the Ni content should be 0.500% or less, preferably 0.300% or less, and more preferably 0.150% or less. ·V: 0.005% or more and 0.100% or less V combines with C and N to form carbonitrides, contributing to improved strength, and is therefore one of the important additive elements in this invention. To obtain this effect, it is necessary to contain 0.005% or more of V. For this reason, the V content is set to 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0034] Furthermore, if the V content exceeds 0.100%, the strength increases, leading to a decrease in ductility, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles to occur when the steel sheet is processed into the curled portion of a can lid. To prevent this, the V content should be 0.100% or less, preferably 0.060% or less, and more preferably 0.040% or less. ·Sn: 0.003% or more and 0.050% or less Sn has the effect of improving hardenability and contributes to improving the pressure resistance strength of the lid, and is therefore one of the important additive elements in this invention. In order to obtain this effect, it is necessary to contain 0.003% or more of Sn. For this reason, the Sn content is set to 0.003% or more, preferably 0.004% or more, and more preferably 0.005% or more.

[0035] Furthermore, if the Sn content is excessive, the proportion of unrecrystallized ferrite will exceed 3%, causing wrinkles to occur when the steel sheet is processed into the curled portion of the can lid. To prevent this, the Sn content should be 0.050% or less, preferably 0.015% or less, and more preferably 0.010% or less. • Mo content as precipitated Mo is 0.013% by mass or less Mo, present in a precipitated state within the steel, contributes to improving the strength of the steel sheet. To obtain this effect, it is preferable that the Mo content as precipitated Mo be 0.001% by mass or more, and more preferably 0.0002% by mass or more, relative to the total mass of all components.

[0036] Furthermore, if the material contains an excessive amount of Mo as precipitated Mo, wrinkles will occur when the steel sheet is processed into the curled portion of the can lid. To prevent this, the Mo content as precipitated Mo should be 0.013% by mass or less relative to the total mass of all components, preferably 0.010% by mass or less, and more preferably 0.008% by mass or less.

[0037] The steel sheet for cans in this embodiment has a composition in which the above-mentioned elemental components are essential, with the remainder being iron and unavoidable impurities.

[0038] Inevitable impurities are impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included to the extent that they do not hinder the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of impurities include O, H, Zn, Pb, As, Sb, Bi, or Co. [Metal structure] Next, we will explain the appropriate range and reasons for limiting the metallographic structure, which is an important requirement for the steel sheet used for cans in this embodiment. In the following, "area ratio" refers to the area ratio relative to the entire steel sheet structure. • Percentage of ferrite phase: 70% or more by area ratio Ferrite is formed during cooling after annealing and contributes to improving the ductility of steel. If the proportion of the ferrite phase is less than 70% by area, it becomes difficult to achieve the desired ductility. Therefore, the proportion of the ferrite phase should be 70% or more by area, preferably 71% or more.

[0039] Furthermore, if the proportion of the ferrite phase is 99.5% or less by area ratio, it becomes easier to maintain a uniform shape in the steel sheet. Therefore, it is preferable that the proportion of the ferrite phase be 99.5% or less by area ratio.

[0040] Note that the proportion of the ferrite phase referred to here does not include the proportion of unrecrystallized ferrite, which will be discussed later. • Proportion of martensite phase: 5% to 22% by area Martensite has the effect of increasing the yield strength YS (MPa) and tensile strength TS (MPa) of the steel sheet, thereby improving the pressure resistance of the can lid. If the proportion of the martensite phase is less than 5% by area, the increase in the strength of the steel sheet will be small, and the pressure resistance of the can lid will be low. Therefore, the proportion of the martensite phase should be 5% or more by area, preferably 6% or more, and more preferably 7% or more.

[0041] Furthermore, if the proportion of the martensite phase exceeds 22% by area, the steel sheet will be excessively strengthened, and the total elongation will decrease. To prevent this, the proportion of the martensite phase should be 22% or less by area, preferably 21% or less, and more preferably 20% or less. • Percentage of unrecrystallized ferrite: 3% or less as a percentage of the total tissue area. If the proportion of unrecrystallized ferrite exceeds 3% in terms of area relative to the total structure, wrinkles will occur when the steel sheet is processed into the curled portion of the can lid. Therefore, the proportion of unrecrystallized ferrite should be 3% or less in terms of area relative to the total structure. The mechanism by which wrinkles occur is not clear, but it is thought that if a large amount of unrecrystallized ferrite is present, the processing stress concentrates at the locations where the unrecrystallized ferrite is present during the curling process, leading to the formation of wrinkles. Preferably, the proportion of unrecrystallized ferrite should be 2.8% or less in terms of area relative to the total structure, and more preferably 2.5% or less.

[0042] Furthermore, if the proportion of unrecrystallized ferrite is to be less than 0.5% in terms of area relative to the total structure, the annealing temperature must be increased, making it impossible to maintain a uniform shape of the steel sheet during annealing. For this reason, it is preferable that the proportion of unrecrystallized ferrite be 0.5% or more, and more preferably 0.8% or more, in terms of area relative to the total structure.

[0043] Furthermore, in the above-described metallographic structure, the remainder other than the ferrite phase and martensite phase does not need to be particularly limited. For example, the remainder may include retained austenite, cementite, pearlite, bainite, etc. Preferably, the remainder is 5% or less. (YS+TS) / 2 ≥ 470MPa In order to ensure the pressure resistance of the can lid, the yield strength YS (MPa) and tensile strength TS (MPa) of the steel sheet for the can in this embodiment must satisfy the relationship shown in equation (1) below.

[0044] (YS+TS) / 2 ≥ 470 MPa ……(1) Furthermore, even if the value of (YS+TS) / 2 exceeds 620 MPa, the pressure resistance strength will saturate, so it is preferable that the value of (YS+TS) / 2 for the steel sheet for cans in this embodiment be 620 MPa or less. • Total growth: 10% or more To ensure the curling properties of the can lid, the total elongation of the steel sheet for the can in this embodiment is set to 10% or more, preferably 12% or more. Furthermore, even if the total elongation exceeds 35%, the curling properties will saturate, so it is preferable that the total elongation of the steel sheet for the can in this embodiment be 35% or less. • Plate thickness: 0.10~0.60mm (optimal conditions) The thickness of the steel sheet for the can in this embodiment is not particularly limited, but it is preferable that the thickness be in the range of 0.10 to 0.60 mm.

[0045] Currently, thinning of steel sheets is being pursued to reduce can manufacturing costs. However, as steel sheets are thinned, i.e., the thickness of the steel sheet is reduced, there are concerns about a decrease in can body strength and forming defects during processing. The present invention aims to prevent a decrease in can body strength, such as the pressure resistance of the can lid, even when the steel sheet thickness is thin, and to prevent forming defects such as wrinkles occurring during the curling process of the can lid. In other words, the effects of the present invention, which are high strength and high processing accuracy, are particularly evident when the steel sheet thickness is thin.

[0046] From this viewpoint, the thickness of the steel sheet for the can in this embodiment is preferably 0.60 mm or less, more preferably 0.40 mm or less, and even more preferably 0.30 mm or less. Furthermore, if the thickness of the steel sheet is 0.10 mm, can manufacturing is easier, so it is preferable that the thickness of the steel sheet for the can in this embodiment be 0.10 mm or more. <Method for manufacturing steel sheets for cans> Next, a method for manufacturing steel sheets for cans according to one embodiment of the present invention will be described below.

[0047] In the following explanation, temperature is based on the surface temperature of the steel plate. The average cooling rate is the average value obtained by calculating the starting temperature, ending temperature, and cooling time from the starting temperature to the ending temperature, based on the surface temperature of the steel plate. For example, the average cooling rate from 500°C to 300°C is expressed as {(500°C)-(300°C)} / (cooling time from 500°C to 300°C).

[0048] When manufacturing steel sheets for cans according to this embodiment, molten steel is adjusted to the above-mentioned component composition by known methods such as converters and electric furnaces, and then formed into slabs by, for example, continuous casting.

[0049] The method for melting the slab having the above-mentioned component composition is not particularly limited. Any known melting method, such as an electric furnace or converter, can be used, and secondary refining may be performed in a vacuum degassing furnace. Subsequently, from the viewpoint of productivity and quality stability, it is preferable to manufacture the slab (steel material) by a continuous casting method, but the slab may also be manufactured by a known casting method such as the ingot-parting rolling method. Since the steel sheet for cans according to this embodiment has Cu, Ni, and Sn as essential elements, it is possible to manufacture it without going through the iron ore reduction process by melting iron scrap containing these three elements in an electric furnace, which is also beneficial from the viewpoint of carbon neutrality and resource recycling.

[0050] The manufacturing method for steel sheets for cans according to this embodiment includes a hot rolling step, a cold rolling step, an annealing step, and a temper rolling step. In the hot rolling step, a slab having the above-mentioned component composition is heated to 1150°C or higher and hot-rolled under conditions that the finishing temperature is 800°C to 950°C and the coiling temperature is 550°C to 750°C. In the cold rolling step, the hot-rolled sheet that has gone through the hot rolling step is cold-rolled to a reduction ratio of 85% or higher. In the annealing step, the cold-rolled sheet that has gone through the cold rolling step is heated to an annealing temperature of 640°C to 860°C at an average heating rate of 5 to 40°C / second. Then, after holding at this annealing temperature for 10 to 90 seconds, the sheet is first cooled to a cooling stop temperature of 250 to 350°C at an average cooling rate of 10°C / sec to 230°C / sec, and then secondarily cooled to a cooling stop temperature of 150 to 250°C at an average cooling rate of 0.2°C / sec to 10.0°C / sec. In the temper rolling process, the annealed sheet that has undergone the annealing process is temper-rolled with a temper rolling ratio of 0.5% to 5.0%. [Hot rolling process] • Slab heating temperature: 1150℃ or higher If the slab heating temperature in the hot rolling process is less than 1150°C, the amount of carbonitrides that contribute to the strength of the steel sheet cannot be sufficiently secured after the annealing process, resulting in a decrease in strength. Therefore, the slab heating temperature should be 1150°C or higher, preferably 1160°C or higher. Furthermore, since the effect saturates even if the slab heating temperature exceeds 1350°C, it is preferable to keep the slab heating temperature at 1350°C or lower. • Finishing temperature: 800℃ to 950℃ If the finishing temperature in the hot rolling process exceeds 950°C, the ferrite structure after hot rolling becomes coarser, and consequently, the ferrite structure after the annealing process also becomes coarser, resulting in a decrease in yield strength. For this reason, the finishing temperature should be 950°C or lower, preferably 930°C or lower.

[0051] Furthermore, if the finishing temperature is below 800°C, the proportion of unrecrystallized ferrite will exceed 3% in terms of area relative to the total structure, causing wrinkles to form when the steel sheet is processed into the curled portion of the can lid. To prevent this, the finishing temperature should be 800°C or higher, preferably 830°C or higher. Winding temperature: 550℃ or higher and 750℃ or lower In the manufacturing method of steel sheets for cans according to this embodiment, the desired yield strength is achieved by controlling the winding temperature and the heating rate up to the annealing temperature, thereby precipitating carbonitrides containing Nb and Ti in the ferrite. If the winding temperature exceeds 750°C, sufficient carbonitrides contributing to the yield strength will not precipitate during winding, and furthermore, some of the ferrite in the steel sheet after continuous annealing will become coarser, causing the steel sheet to soften, and the value of equation (1) above, which defines the relationship between yield strength and tensile strength, will fall below 470 MPa. In addition, scale will form on the surface of the steel sheet, making it more susceptible to surface defects. For this reason, the winding temperature should be 750°C or lower, preferably 730°C or lower.

[0052] Furthermore, if the winding temperature is less than 550°C, the proportion of unrecrystallized ferrite in the microstructure of the steel sheet after continuous annealing will exceed 3% in terms of area relative to the total microstructure, causing wrinkles to occur when the steel sheet is processed into the curled portion of a can lid. To prevent this, the winding temperature should be 550°C or higher, preferably 560°C or higher.

[0053] A hot-rolled sheet is obtained through the hot-rolling process described above. [Pickling] After the hot rolling process, pickling is preferably performed as needed. The pickling method should be effective in removing scale from the surface of the steel sheet; there is no need to limit the conditions of the pickling. Alternatively, scale on the surface of the steel sheet may be removed by methods other than pickling. [Cold rolling process] • Reduction ratio in cold rolling: 85% or more By setting the reduction ratio in cold rolling to 85% or more, the strain applied to the steel sheet during the cold rolling process increases, making it possible to set the value of equation (1) above, which defines the relationship between the yield strength and tensile strength of the steel sheet after annealing, to 470 MPa or more. To obtain this effect, the reduction ratio is set to 85% or more, preferably to 86% or more.

[0054] Furthermore, if the reduction ratio exceeds 95%, the rolling load increases significantly, and the load on the rolling mill increases. To avoid this, it is preferable to keep the reduction ratio at 95% or less, and more preferably between 87% and 94%.

[0055] A cold-rolled sheet is obtained through the cold rolling process described above. It is also possible to include other processes, such as an annealing process to soften the hot-rolled sheet, between the hot-rolling and cold-rolling processes. Furthermore, the cold-rolling process may be performed immediately after the hot-rolling process without pickling. [Annealing process] • Average heating rate to annealing temperature: 5-40°C / second In the manufacturing method of steel sheets for cans according to this embodiment, the desired yield strength is achieved by controlling the winding temperature and the heating rate to the annealing temperature, thereby precipitating carbonitrides containing Nb, Ti, and V in the ferrite. To obtain this effect, it is important to control the average heating rate to the annealing temperature to 5 to 40°C / second.

[0056] If the average heating rate is less than 5°C / second, the carbonitride obtained in the hot rolling process will melt during heating, and a sufficient amount of carbonitride cannot be secured after the annealing process. As a result, the ferrite will not be strengthened by precipitates, making it difficult to secure the desired yield strength. To prevent this, the average heating rate to the annealing temperature should be 5°C / second or higher, preferably 8°C / second or higher.

[0057] Furthermore, if the average heating rate exceeds 40°C / second, the area ratio of the ferrite phase will be less than 70%, making it difficult to secure the desired ductility. In addition, the proportion of unrecrystallized ferrite in the steel sheet after the annealing process will exceed 3% in terms of area ratio to the total structure, causing wrinkles to occur when the steel sheet is processed into the curled part of a can lid. To prevent this, the average heating rate to the annealing temperature should be 40°C / second or less, preferably 35°C / second or less. Annealing temperature: 640°C to 860°C If the annealing temperature is below 640°C, the area ratio of the ferrite phase will be less than 70%, making it difficult to ensure the desired ductility. Furthermore, the proportion of unrecrystallized ferrite in the steel sheet after the annealing process will exceed 3% in terms of area ratio relative to the total structure, causing wrinkles to occur when the steel sheet is processed into the curled portion of a can lid. To prevent this, the annealing temperature should be 640°C or higher, preferably 680°C or higher.

[0058] Furthermore, if the annealing temperature exceeds 860°C, precipitates containing Nb, Ti, and V formed during the hot rolling process will dissolve, making it impossible to obtain the desired yield strength after the annealing process. To prevent this, the annealing temperature should be 860°C or lower, preferably 840°C or lower. • Holding time in the temperature range of 640°C to 860°C: 10 seconds to 90 seconds If the holding time at an annealing temperature of 640°C to 860°C is less than 10 seconds, the proportion of unrecrystallized ferrite in the steel sheet after the annealing process will exceed 3% in terms of area relative to the total structure, causing wrinkles to occur when the steel sheet is processed into the curled portion of a can lid. To prevent this, the holding time at an annealing temperature of 640°C to 860°C should be 10 seconds or more, preferably 15 seconds or more.

[0059] Furthermore, if the holding time at an annealing temperature of 640°C to 860°C exceeds 90 seconds, precipitates containing Nb, Ti, and V, which are mainly precipitated during the hot rolling winding process, become coarser during holding at the annealing temperature, reducing the yield strength. To prevent this, the holding time at an annealing temperature of 640°C to 860°C should be 90 seconds or less, preferably 50 seconds or less.

[0060] A continuous annealing apparatus can be used for the annealing process. Other processes may be included between the cold rolling process and the annealing process as appropriate, or the annealing process may be performed immediately after the cold rolling process. • Average cooling rate of primary cooling: 10°C / sec to 230°C / sec If the average cooling rate from the annealing temperature to the cooling stop temperature is less than 10°C / second, precipitates containing Nb, Ti, and V in the steel sheet after the annealing process become coarser during cooling, reducing the yield strength. To prevent this, the average cooling rate from the annealing temperature to the cooling stop temperature should be 10°C / second or higher, preferably 15°C / second or higher. Cooling from the annealing temperature to the cooling stop temperature can be carried out by gas cooling, or by a combination of one or more methods such as mist cooling, roll cooling, and water cooling.

[0061] Furthermore, if the average cooling rate from the annealing temperature to the cooling stop temperature exceeds 230°C / second, the proportion of the martensite phase in the steel sheet after the annealing process will exceed 22%, and the proportion of the ferrite phase will be less than 70%, resulting in reduced ductility and the formation of wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the average cooling rate from the annealing temperature to the cooling stop temperature should be 230°C / second or less, preferably 200°C / second or less. • Primary cooling stop temperature: 250°C or higher and 350°C or lower If the cooling stop temperature for primary cooling is set below 250°C, the proportion of martensite phase in the steel sheet after the annealing process will exceed 22%, and the proportion of ferrite phase will be less than 70%, resulting in reduced ductility and the formation of wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the cooling stop temperature should be set at 250°C or higher, preferably 270°C or higher.

[0062] Furthermore, if the cooling stop temperature for primary cooling exceeds 350°C, wrinkles will occur when the steel plate is processed into the curled part of the can lid. To prevent this, the cooling stop temperature The temperature should be 350°C or lower, preferably 330°C or lower. • Average cooling rate of secondary cooling: 0.2°C / sec to 10.0°C / sec If the average cooling rate from the primary cooling stop temperature to the secondary cooling stop temperature is less than 0.2°C / second, precipitates containing Nb, Ti, and V in the steel sheet after the annealing process become coarser during cooling, reducing the yield strength and decreasing the pressure resistance when the steel sheet is processed into a can lid. To prevent this, the average cooling rate from the annealing temperature to the cooling stop temperature should be 0.2°C / second or higher, preferably 0.3°C / second or higher. Cooling from the primary cooling stop temperature to the secondary cooling stop temperature can be carried out by gas cooling, or by a combination of one or more methods such as mist cooling, roll cooling, and water cooling.

[0063] Furthermore, if the average cooling rate from the cooling stop temperature of the primary cooling to the cooling stop temperature of the secondary cooling exceeds 10.0°C / second, the proportion of the martensite phase in the steel sheet after the annealing process will exceed 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the average cooling rate from the cooling stop temperature of the primary cooling to the cooling stop temperature of the secondary cooling should be 10.0°C / second or less, preferably 5.0°C / second or less. • Secondary cooling shutdown temperature: 150°C to 200°C If the cooling stop temperature for secondary cooling is set below 150°C, the proportion of the martensite phase in the steel sheet after the annealing process exceeds 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the cooling stop temperature for secondary cooling should be set at 150°C or higher, preferably 155°C or higher.

[0064] Furthermore, if the cooling stop temperature for secondary cooling exceeds 200°C, precipitates containing Nb, Ti, and V in the steel sheet after the annealing process become coarser during cooling, reducing the yield strength and decreasing the pressure resistance when the steel sheet is processed into a can lid. To prevent this, the cooling stop temperature for secondary cooling should be 200°C or lower, preferably 195°C or lower.

[0065] After the annealing process described above, an annealed plate is obtained. [Temper rolling process] ·Temper rolling ratio: 0.5% or more and 5.0% or less Temper rolling allows for adjustment of the surface roughness and correction of the shape of the steel sheet, as well as improvement of the yield strength by introducing strain into the annealed sheet. To achieve this effect, the temper rolling ratio must be 0.5% or higher. Therefore, the temper rolling ratio should be 0.5% or higher, preferably 0.6% or higher.

[0066] Furthermore, if the temper rolling ratio exceeds 5.0%, excessive strain is introduced into the steel sheet, reducing its ductility and causing wrinkles to form when the steel sheet is processed into the curled portion of the can lid. To prevent this, the temper rolling ratio should be 5.0% or less, preferably 2.0% or less.

[0067] The steel sheet for cans of the present invention is obtained through the temper rolling process described above.

[0068] Furthermore, in the manufacturing method of steel sheets for cans according to the present invention, various processes may be carried out after the temper rolling process. For example, the steel sheets for cans according to the present invention may have a plating layer on the surface of the steel sheet. Examples of plating layers include a Sn plating layer, a tin-free Cr plating layer, a Ni plating layer, a Sn-Ni plating layer, etc. Therefore, a plating process may be carried out after the temper rolling process to form a plating layer on the surface of the steel sheet. Alternatively, a paint baking process, a film lamination process, etc. may be carried out after the temper rolling process. Note that since the film thickness of the plating layer, laminate film, etc. is sufficiently small compared to the thickness of the steel sheet, its influence on the mechanical properties of the steel sheets for cans can be ignored. [Examples]

[0069] Examples of the steel sheet for cans and its manufacturing method according to the present invention will be described below. However, the steel sheet for cans and its manufacturing method according to the present invention are not limited to the examples shown below.

[0070] Steel containing the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and continuously cast to obtain steel slabs, which were the raw material for the steel. Next, these steel slabs were hot-rolled under the hot-rolling conditions shown in Tables 2 and 3, and then pickled after hot-rolling. Furthermore, cold-rolling was performed at the reduction ratios shown in Tables 2 and 3, continuous annealing was performed under the annealing conditions shown in Tables 2 and 3, and temper-rolling was performed at the reduction ratios shown in Tables 2 and 3 to obtain steel plates.

[0071] The steel sheet obtained in this manner was then continuously plated with conventional Sn plating, resulting in a plating layer adhesion amount of 11.2 g / m² on one side. 2 A Sn-plated steel sheet (tinplate) was obtained. Then, at 210°C, Sn-plated steel sheets that had undergone heat treatment equivalent to a 10-minute paint baking process were used as the test subjects, and the following test items were evaluated. <Tensile Test> Tensile tests were conducted on the Sn-plated steel sheets under test in accordance with the test method specified in Japanese Industrial Standard JIS Z 2241:2011 "Tensile Test Method for Metallic Materials," and the yield strength (YS), tensile strength (TS), and total elongation were measured. Specifically, JIS No. 5 tensile test specimens were taken using the method specified in Japanese Industrial Standard JIS Z 2201 "Tensile Test Specimens for Metallic Materials," so that the direction parallel to the rolling direction was the tensile direction. Then, a gauge mark of 50 mm (L) was applied to the parallel section of the tensile test specimen, and a tensile test was conducted at a tensile speed of 10 mm / min until the tensile test specimen fractured, in accordance with the provisions of JIS Z 2241. In this invention, yield strength (YS) refers to the upper yield point if there is one, or the 0.2% proof stress if there is no upper yield point. The measurement results of this yield strength are shown in Tables 2 and 3. <Investigation of metal structure> The metallographic structure of the Sn-plated steel sheet under test was investigated by polishing a cross-section parallel to the rolling direction in the thickness direction, then etching this cross-section with an etching solution (3 vol% nital), and observing it with a scanning electron microscope (SEM). Specifically, using an SEM at 1000x magnification, five randomly selected fields of view were observed, covering the region from a depth of 1 / 4 of the sheet thickness (1 / 4 of the sheet thickness in the thickness direction from the surface of the steel sheet in the cross-section described above) to a depth of 1 / 2 of the sheet thickness.

[0072] Then, for each field of view, the unrecrystallized ferrite in the metal structure was identified by visual inspection using images taken with a SEM, and the proportion of unrecrystallized ferrite (area ratio to the total structure) was determined by image analysis. Here, unrecrystallized ferrite is a metal structure that exhibits an elongated shape in the rolling direction when observed at 1000x magnification with a SEM. The average value of the area ratio of unrecrystallized ferrite to the total structure obtained in this way at five locations in the field of view was defined as the area ratio of unrecrystallized ferrite to the total structure. Image analysis software (particle analysis, manufactured by Nippon Steel & Sumitomo Metal Technology Co., Ltd.) was used for the above image analysis.

[0073] Furthermore, the area fractions of the ferrite phase and martensite were investigated using the same method as for unrecrystallized ferrite. Specifically, the cross-section of the Sn-plated steel sheet under test was polished in the thickness direction parallel to the rolling direction, and then this cross-section was etched with an etching solution (3 vol% nital). The area fractions of the ferrite phase and martensite were then investigated by observation with a scanning electron microscope (SEM). In particular, using an SEM, at a magnification of 2000x, three randomly selected fields of view were observed at a depth of 1 / 2 the thickness of the sheet.

[0074] Then, for each field of view, the area ratio of the ferrite phase and the area ratio of martensite within a randomly set 50 μm × 50 μm square area was determined by binarization using image processing software (Photoshop, Adobe). The average values ​​of the area ratio of the ferrite phase and the area ratio of martensite obtained in this way for the three fields of view were defined as the area ratio of ferrite and the area ratio of martensite, respectively. In the SEM observation, black areas observed as clumps were identified as ferrite, and white areas were identified as martensite.

[0075] The results of the investigation into this metallographic structure are shown in Tables 2 and 3. <Amount of precipitated Mo> After removing the Sn plating from the Sn-plated steel sheets under test by pickling, precipitates were extracted using a 10% AA electrolyte, filtered and collected, and then subjected to mixed acid decomposition. The amount of precipitated Mo was then measured by ICP-AES (inductively coupled plasma atomic emission spectroscopy). The results of this investigation are shown in Tables 2 and 3. <Corrosion resistance> For the Sn-plated steel sheet being tested, an optical microscope was used to measure an area of ​​2.7 mm². 2 Measurement area The samples were observed at 50x magnification, and the number of hole-like areas where the Sn plating was thinned was counted. The results of this corrosion resistance investigation are shown in Tables 2 and 3. In Tables 2 and 3, "○" indicates fewer than 20 hole-like areas, "△" indicates 20 to 25 areas, and "×" indicates more than 25 areas. <Presence or absence of wrinkles in the curled area> A blank was taken from the Sn-plated steel sheet under test, and can lids were fabricated by sequentially performing shell processing and curl processing. The curled portion of the fabricated can lids was then visually observed at eight locations around the circumference of the lid to check for the presence or absence of wrinkles. The results of this investigation are shown in Tables 2 and 3. In Tables 2 and 3, "Wrinkle occurrence: Present" is used if wrinkles occurred at at least one of the eight locations around the circumference, and "Wrinkle occurrence: Absent" is used if no wrinkles occurred at any of the eight locations around the circumference. <Pressure resistance strength> A rectangular blank was taken from the Sn-plated steel sheet under test, and a can lid was fabricated by sequentially performing shell processing and curling. Then, a pressure resistance strength test was conducted using this can lid as the test specimen.

[0076] The pressure resistance test was conducted using a CMC-KUHNKE buckle tester BCL-3153L under test conditions of an initial pressure of 0.5 bar and a pressure increase rate of 0.3 bar / min. The pressure was increased from one side of the can lid while the curled portion of the lid was fixed. The pressure value at which the can lid deformed was defined as the pressure resistance value. The results of this pressure resistance test are shown in Tables 2 and 3. In Tables 2 and 3, a pressure resistance value of 1.3 bar or higher is indicated by "○", and a pressure resistance value of less than 1.3 bar is indicated by "×".

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] Steel plates No. 1-38, 63, 78, 81, 86, 87, and 89, which are examples of inventions that satisfy the requirements of the steel plate for cans and the method for manufacturing the same according to the present invention, were confirmed to have excellent processability and pressure resistance of the curled portion of the can lid while possessing the corrosion resistance necessary for steel plates for cans.

[0081] In contrast, steel plates No. 39-61, 62, 64-77, 79, 80, 82-85, 88, and 90-105 are comparative examples that do not satisfy any of the requirements for the steel plate for cans and its manufacturing method of the present invention. These steel plates No. 39-61, 62, 64-77, 79, 80, 82-85, 88, and 90-105 failed to achieve the desired level in terms of corrosion resistance, processability of the curled portion of the can lid, or compressive strength, and their performance was significantly inferior to that of the steel plate in the invention example.

Claims

1. In mass percent, C: 0.05% or more and 0.13% or less, Si: 0.01% or more and 0.04% or less, Mn: 0.1% or more and 0.6% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.01% or more and 0.10% or less, N: 0.0005% or more and 0.0080% or less, Ti: 0.005% or more and 0.030% or less, Nb: 0.005% or more and 0.030% or less, Mo: 0.010% or more and 0.050% or less, B: 0.0005% or more and 0.0050% or less, Cu: 0.003% or more and 0.500% or less, Cr: 0.003% or more and 0.300% or less, Ni: 0.005% or more and 0.500% or less, V: 0.005% or more and 0.100% or less, Sn: 0.003% or more and 0.050% or less, It contains, with the remainder being iron and unavoidable impurities, and has a component composition. The Mo content of precipitated Mo among the aforementioned Mo is 0.013% by mass or less. The metal structure contains a ferrite phase of 70% or more by area ratio, a martensite phase of 5% to 22% by area ratio, and the proportion of unrecrystallized ferrite is 3% or less by area ratio of the total structure. The yield strength YS (MPa) and tensile strength TS (MPa) are given by the following equation (1) (YS+TS) / 2 ≧470MPa...(1) Satisfying the relationship, Steel sheet for cans with a total elongation of 10% or more.

2. A method for manufacturing steel sheets for cans according to claim 1, A hot rolling process is performed on a steel slab having the above-mentioned component composition, which is heated to 1150°C or higher, and hot-rolled under conditions that the finishing temperature is 800°C to 950°C and the winding temperature is 550°C to 750°C. A cold rolling process is performed on the hot-rolled sheet that has undergone the aforementioned hot-rolling process, in which a reduction ratio of 85% or more is achieved. An annealing process is performed in which the cold-rolled sheet that has undergone the cold-rolling process is heated to an annealing temperature of 640°C to 860°C at an average heating rate of 5 to 40°C / second, held at the annealing temperature for 10 to 90 seconds, primary cooling is performed to a cooling stop temperature of 250°C to 350°C at an average cooling rate of 10°C to 230°C / second, and secondary cooling is performed to a cooling stop temperature of 150°C to 200°C at an average cooling rate of 0.2°C to 10.0°C / second. A method for manufacturing steel sheets for cans, comprising: a temper rolling step of subjecting the annealed sheet that has undergone the aforementioned annealing step to temper rolling with a temper rolling ratio of 0.5% or more and 5.0% or less.

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